1. Field of the Invention
The present invention relates to a method and an apparatus for manufacturing a semiconductor device and, more particularly, to a method of measuring a film thickness of a resist film coated on a semiconductor wafer.
2. Description of the Related Art
In manufacturing a semiconductor device such as an IC or an LSI, a plurality of photolithography operations are performed on a semiconductor wafer to form a large number of chips having electrodes in a predetermined pattern.
Along with an increase in integration degree of the electrodes on a device, it is required to accurately and uniformly form a resist layer (film) having a predetermined thickness on a semiconductor wafer. In order to satisfy such a requirement, a spin coater which can suppress a variation in thickness of a resist film to fall within a range of 4 to 5 nm has been developed and is commercially available.
When a resist film is formed by a spin coater, the thickness of the resist film varies unless the temperature and humidity of a resist solution, a wafer, and a coating atmosphere are constant. Even in a single wafer, the film thickness tends to be increased at a central portion, and:tends to be decreased at a peripheral portion. If the thickness of the resist film is uneven, a width of an electrode varies, and a micro-pattern cannot be accurately formed by photolithography.
In a conventional system, the thickness of a resist film of a wafer to be a product is not directly measured in a production process, but only the thickness of a dummy wafer (sampling wafer) is measured. However, in a semiconductor device (e.g. super LSI) with an ultra-micropattern, since a variation in thickness of the resist film seriously influences the width of an electrode, 10 the thickness of the resist film coated on a wafer to be a product must be measured in a production process.
In the step of forming a large number of semiconductor chips on a wafer, a plurality of photolithography steps are performed. In these photolithography steps, a wafer undercoating layer is in a flat state often in only a first photolithography step.
In a conventional method of measuring the thickness of a resist film, light of a predetermined wavelength is radiated on a substrate surface, light components reflected by the substrate and resist film surfaces are respectively detected, and a film thickness is calculated and determined on the basis of the detected light components.
However, in the conventional method of measuring the thickness of the resist film, if the undercoating layer of the wafer is not flat, the thickness of the resist film cannot be accurately measured. For this reason, in the photolithography steps, except for the first photolithography step, the resist film thickness cannot be measured on an arbitrarily selected point on a wafer. Especially, in the step of coating a resist solution, since the thickness of the resist film of the wafer varies due to the influence of the environmental temperature and humidity, the thickness of the resist film cannot be accurately determined. Thus, a width of an electrode may vary, and the manufacturing yield of semiconductor devices with ultra-micropatterns is decreased.